1. Field of the Invention
The present invention relates to a sensor-less driving technique of detecting a position of a magnetic pole of a rotor with not a position detecting means such as a Hall elements but an induced voltage developed in windings of a stator. Particularly, the invention relates to an apparatus and a method for driving a brushless motor through detecting the magnetic pole position of the rotor without the use of a sensor, in which a circulating current period of an inverter is determined from the terminal voltage at a phase which is not energized (non-energized phase) and the DC voltage applied to the main line of the inverter, and the terminal voltage after the inverter circulating current period and the waveform of the terminal voltage predetermined from the characteristics of the brushless motor are compared to determine the position of the rotor.
2. Description of the Related Art
For controlling the rotation of a brushless motor, it is essential to allow an action of commutation with the relation between the magnetic pole position of the rotor and the winding to be energized. Output torque of the brushless motor is determined by an interaction of the force of magnetic flux of magnets mounted to the rotor and the force of magnetic flux of a current flown through the windings of the stator. It is hence necessary for driving and controlling the rotation of the brushless motor to feed a current to a particular position on the windings where the magnetic flux developed by the magnetic poles of the rotor is maximum and thus generate an optimum force of torque. Also, the rotation of the brushless motor can be controlled by shifting a phase to be energized (energized phase) from one to another along the magnetic pole position of the rotor. When the timing of shifting or commutation is lagged greatly from the location at the maximum magnetic flux, the force of torque will be declined. In the worst case, the rotation of the brushless motor will be out of tune and finally stalled.
Therefore, the rotation of the brushless motor has to be controlled through detecting exactly the magnetic pole position of the rotor with the use of any means. Some sensor-less driving methods have been proposed for detecting the magnetic pole position of the rotor with the use of not a position detecting means such as a Hall element but measurement of an induced voltage developed in the windings of the stator. One of the conventional methods of detecting the magnetic pole position of the rotor with no use of a sensor is illustrated in the form of a system in FIGS. 23 and 24.
The system shown in FIG. 23 comprises an alternating current source 1, a converter 2, an inverter 3, a brushless motor 5, a stator 6, a rotor 7, a controller 8, a drive circuit 9, a reference voltage generator 16, and a group of comparators 17u, 17v and 17w. The brushless motor 5 includes the stator 6 having three phase windings 6u, 6v and 6w connected to each other in a Y connection about an neutral point and the rotor 7 equipped with magnets. The U-phase winding 6u, V-phase winding 6v, and W-phase winding 6w are connected at their free ends to a U-phase terminal 11u, a V-phase terminal 11v, and a W-phase terminal 11w respectively.
An AC voltage released from the alternating current source 1 is converted by the converter 2 into a DC voltage (Vdc) which is then transferred to the inverter 3. The inverter 3 comprises three series circuits for U-phase, V-phase, and W-phase, each circuit having a pair of switching elements connected between the upstream and the downstream in the current flow. The three series circuits are fed with a DC voltage (Vdc) from the converter 2. The U-phase series circuit includes a pair of transistors 12u and 13u as the upstream switching element and the downstream switching element respectively. Equally, the V-phase series circuit includes a pair of transistors 12v and 13v as the upstream switching element and the downstream switching element respectively while the W-phase series circuit includes a pair of transistors 12w and 13w as the upstream switching element and the downstream switching element respectively. The free-wheel diodes 14u, 14v, 14w and 15u, 15v, 15w are connected in parallel with the transistors at both the upstream and the downstream.
In the inverter 3, a connecting node between transistors 12u and 13u is connected to a terminal 11u of the brushless motor 5. Connecting nodes between the transistors 12v and 13v, and 12w and 13w are connected to corresponding terminals 11v and 11w, respectively. The inverter 3 turns on and off the transistors in a sequence to energize the phase windings 6u, 6v, and 6w of the brushless motor 5. While the non-energized period is provided for disconnecting the paired transistors at the upstream and downstream at once, the magnetic pole position of the rotor 7 is detected.
Each of the comparators 17u, 17v and 17w compares the terminal voltage (induced voltage) developed at the corresponding terminal 11u, 11v, or 11w of the brushless motor 5 with a reference voltage (e.g. a half of the DC voltage Vdc) from reference voltage generator 16. A resultant signal which changes at the intersection is released as the position detecting signal to the controller 8 as shown in FIG. 24. The controller 8 generates a set of control signals (u+, v+, w+, uxe2x88x92, vxe2x88x92, and wxe2x88x92) for energizing in a sequence the phase winding 6u, 6v, and 6w of the brushless motor 5 on the basis of the change point of the position detecting signal. The control signals are fed into the drive circuit 9. In this manner, the rotation of the brushless motor 5 can be controlled.
Another example of the conventional sensor-less driving method is disclosed in Japanese Patent No.2786863. That method employs an A/D converter for sampling the terminal voltage at the non-energized phase of a brushless motor, determining the gradient of the induced voltage from two samples, and performing an action of commutation from the intersection between the gradient and a half of the DC voltage.
In the above described conventional method, it is however essential to have the intersection between the induced voltage and the reference voltage during the non-energized period. This will limit the action of controlling the brushless motor over the energized period. More specifically, the energized period should be within an angle of 120 degrees and the energization at an angle greater than 120 degrees will hardly be permitted.
The system disclosed in Japanese Patent No.2786863 requires two or more samples of the terminal voltage at the non-energized phase, which has the following problem. When the brushless motor is rotated at a higher speed and two or more samples of the terminal voltage can not be detected, no gradient of the induced voltage can not be calculated. Thus, no timing of commutation is determined, and hence the movement of the brushless motor stalls.
It is an object of the present invention to provide a driving control scheme for controlling the rotation of a brushless motor throughout its range from low high speeds through accurately identifying the magnetic pole position of a rotor, thereby eliminating the above disadvantages.
A brushless motor driving apparatus according to the present invention is an apparatus for driving a brushless motor which includes a stator having a multiple-phase windings and a rotor having multiple-pole magnets. The apparatus detects a magnetic pole position of the rotor and switching the winding of the stator to be energized seriatim according to the detected magnetic pole position through an inverter. The apparatus comprises a DC voltage detector, a terminal voltage detector, a circulating current period determining section, a memory, and a magnetic pole position detector. The DC voltage detector detects a DC voltage applied to a main line of the inverter. The terminal voltage detector detects a terminal voltage in a non-energizing phase of the windings of the stator. The circulating current period determining section determines a circulating current period for which a circulating current is flowing according to the DC voltage and the terminal voltage. The memory stores data of a waveform of the terminal voltage predetermined from characteristics of the brushless motor. The magnetic pole position detector identifies the magnetic pole position of the rotor based on the terminal voltage after the end of the circulating current period and the waveform of a terminal voltage predetermined from characteristics of the brushless motor.
A brushless motor driving method according to the present invention is a method of driving a brushless motor which includes a stator having a multiple-phase windings and a rotor having multiple-pole magnets. The method comprises detecting a magnetic pole position of the rotor and switching the winding of the stator to be energized seriatim according to the detected magnetic pole position through an inverter. The method comprises detecting a DC voltage applied to a main line of the inverter, detecting a terminal voltage in a non-energizing phase of the windings of the stator, determining a circulating current period for which a circulating current is flowing according to the DC voltage and the terminal voltage, and identifying the magnetic pole position of the rotor based on the terminal voltage after the end of the circulating current period and a waveform of a terminal voltage predetermined from characteristics of the brushless motor.
The present invention allows the magnetic pole position of the rotor to be identified without calculating the intersection between the DC voltage and a reference voltage, e.g. xc2xd the DC voltage, thus stably controlling the rotation of the brushless motor throughout its range from low speeds to high speeds.